The present invention is directed generally to broadband microwave filters, and in particular, to broadband recursive microwave filters that provide sharp transitions in the frequency domain between adjacent stopband passbands.
There is a need for microwave filters which have sharp transitions in the frequency domain between adjacent stopbands and passbands. This need is especially acute for very wideband signal systems which operate by chopping the wideband signal up into a series of contiguous smaller bandwidths, and then processing these individual smaller bandwidths. This type of wideband signal chopping into smaller contiguous bandwidths is required, for instance, in monolithic full-band millimeter-wave receivers for processing the ultra-wideband microwave I.F. signals that result. Typically, this signal chopping leads to individual smaller bandwidths of octave (or near-octave) bandwidths. However, if the transition between individual contiguous octave filters is not sharp in the frequency domain, then spurious frequency responses will be generated in a particular octave filter by signals adjacent to but not within the particular octave filter band. Such spurious signals are sometimes referred to by the term frequency aliasing.
At very low frequencies, active filters are commonly utilized in order to obtain the higher order transfer functions needed to provide sharp transitions between stopbands and passbands. Such active filters are advantageous at these low frequencies because they have the ability not only to compensate for parasitic losses affiliated with passive circuit elements, but also to provide overall amplification. However, the direct transposition of low-frequency design principles to the microwave range is impeded by the lack of appropriate broadband, high-gain devices to perform operational amplifier functions. Additionally, a principal limiting factor of current microwave active devices is that such active devices have an intrinsic time delay. Accordingly, most of the interest in microwave active filters has concentrated on alternate approaches in which individual reactances and resonators are replaced with microwave active substitutes that yield higher-Q performance.